Deposition of γ-Al2O3 by means of CVD

ABSTRACT

There is disclosed a coated body having as the outer layer a layer of γ-Al 2 O 3  deposited by chemical vapor deposition, preferably at a temperature of from 700-900° C. The γ-Al 2 O 3  layer is formed through the use of a gaseous mixture including H 2 S in amounts significantly higher than those presently used and at a temperature of from 700-900° C. The method is also disclosed.

BACKGROUND OF THE INVENTION

Aluminum oxide (alumina, Al₂O₃) exists in many metastable polymorphs such as γ, η, θ, δ, κ, and χ in addition to the thermodynamically stable α-Al₂O₃ phase (corundum). When produced by chemical vapor deposition (CVD), Al₂O₃ crystallizes into κ-Al₂O₃ and θ-Al₂O₃ modifications in addition to the stable α-Al₂O₃. Earlier CVD coatings were usually mixtures of several polymorphs, the most commonly occurring metastable Al₂O₃ modification being κ-Al₂O₃. Today, both α-Al₂O₃ and κ-Al₂O₃ polymorphs are used as hard coatings, and they can be deposited in a controlled way by modern CVD technology shown, for example in U.S. Pat. Nos. 5,137,774 and 5,700,569.

Dopants of hydrogen sulfide (H₂S), phorphorus chloride (PCl₃) carbonoxysulfide (COS) or phosphine (PH₃) can be applied in order to increase the growth rate and thickness uniformity of the alumina layers. The most commonly applied dopant is H₂S as also disclosed in U.S. Pat. No. 4,619,886. While amounts of H₂S in the total CVD gaseous mixture of from 0.003 to 1% by volume and temperatures of 700 to 1200° C. are broadly disclosed in that patent, all exemplifications of the process therein are below 0.5 vol % and generally around 0.1 to 0.3 vol % H₂S used at temperatures of 1000-1030° C. H₂S has been called the “magical dopant” in view of its effect on improving the growth rate and uniformity of Al₂O₃ coatings applied by conventional CVD techniques at temperatures around 980° C. See, Oshika et al., “Unveiling the Magic of H₂S on the CVD-Al₂O₃ Coating”, J. Phys IV France 9 (1999), Pr 8-877-Pr 8-883.

CVD κ-Al₂O₃ is considered to exhibit morphological advantages (smaller grain size and lower porosity), lower thermal conductivity and even a higher hardness when compared with the CVD α-Al₂O₃ phase. These are important properties when metal cutting is concerned. However, at the relatively high temperatures (>1000° C.) reached during metal cutting, metastable κ-Al₂O₃ may transform to the stable α-Al₂O₃ polymorph. γ-Al₂O₃, when deposited using physical vapor deposition (PVD) or plasma assisted CVD has been found to exhibit high hardness and good wear properties. See, for example, WO 9924634 and U.S. Pat. No. 5,879,823. However, γ-Al₂O₃ has not been available using conventional CVD techniques.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to avoid or alleviate the problems of the prior art.

It is further an object of this invention to provide γ-Al₂O₃ using conventional CVD.

In one aspect of the invention there is provided a coated body having as the outer layer, a layer of γ-Al₂O₃ deposited by chemical vapor deposition.

In another aspect of the invention there is provided a method of forming a coated body having a layer of γ-Al₂O₃ comprising coating the body with a gaseous mixture of AlCl₃, CO₂, H₂ and H₂S at a temperature of from about 600 to 800° C., the H₂S being present in amounts of at least 0.7% of the total mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a graph of growth rates of κ and γ-Al₂O₃ at varying temperatures and amounts of H₂S in the gaseous coating mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

It has hitherto been thought that γ-Al₂O₃ can only be obtained by plasma assisted CVD or PVD processes. It has now surprisingly been found that γ-Al₂O₃ can be obtained by using conventional CVD under specific circumstances discussed below.

Growth rates of α-alumina vs. H₂S content at 1000° C. and 800° C. are shown in The FIGURE. An important observation is that at 800° C., reasonably high growth rates could be obtained. The most important observation, however, is that at higher H₂S contents, γ-Al₂O₃ was obtained. At 800° C. γ-Al₂O₃ could be obtained at H₂S contents of 0.75-1.7% by volume of the total gaseous mixture, preferably greater than 1 vol %, and at a pressure of 100 mbar. At 1000° C., regardless of the amounts of H₂S, κ-Al₂O₃ is always formed. Thus, by carefully controlling the amount of H₂S and the temperature of the application, the preferred polymorph, γ-alumina, can surprisingly be formed. When studied using TEM (transmission electron microscopy), γ-Al₂O₃ exhibited a very high defect density and obviously a high hardness.

The product of the present invention may be made utilizing conventional CVD techniques and apparatus, using, however, a greater amount of H₂S than conventionally used in CVD processes and a higher pressure. H₂S is added in amounts greater than 0.7 vol %, generally 0.75 to 1.7 vol %, preferably greater than 1 up to about 1.5 vol %, of the total gaseous mixture. The amounts of the other reactants can be adjusted accordingly but usually the alumina formers, an aluminum halide and an oxidizing gas (e.g., water vapor formed by the reaction of CO₂ and/or CO and H₂) are maintained as before and the amount of reducing agent (excess H₂) is reduced.

The coating process is performed at temperatures of from about 700 to 900° C., preferably 750 to 850° C., at a pressure of from about 50 to 600 mbar, preferably from about 100 to 300 mbar, for a time sufficient to form the coating, generally from about 2 to 10 hours, preferably from about 4 to 8 hours.

The resulting coating is from about 1.0 to 5 μm, preferably from about 2 to 4 μm, in thickness.

The body on which the γ-Al₂O₃ layer is applied can be a cemented carbide, ceramic, cermet (for metal cutting purposes) or steel (for catalysis). These bodies are well-known in the art and any such conventional material may be used.

The γ-Al₂O₃ layer may be applied as the outermost or as an inner layer. When used as the outermost layer, the γ-Al₂O₃ layer may be applied onto an Al₂O₃ layer, which itself can be applied onto one or more other layers such as, for example, TiC and (Ti,Al)N. The Al₂O₃ layer can be an alpha phase, a kappa phase or a mixture of alpha and kappa phase Al₂O₃. The γ-Al₂O₃ layer may also be applied onto a TiN, Ti(C,N) or (Ti,Al)N layer.

Similarly, when the γ-Al₂O₃ layer is applied as an inner layer, there may be other layers such as Al₂O₃, TiC, Ti(C,N), TiN or the like applied atop the γ-Al₂O₃ layer.

These various inner and or outer layers may be applied by CVD, MTCVD or PVD.

The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.

EXAMPLE 1

A single layer of γ-Al₂O₃ was deposited using the following process data on a Ti(C,N) layer having a thickness of 3 μm:

T=800° C.

H₂S=1%

AlCl₃=2.5%

CO₂=5.0%

H₂=balance

Pressure (P)=100 mbar

Deposition time is 6 hours

A coating composed of γ-Al₂O₃ was obtained. The same coating was also deposited atop PVD TiN, Ti(C,N) and (Ti,Al)N layers with a thickness of 3 μm. Further, CVD coatings of κ-Al₂O₃ and α-Al₂O₃ were deposited on the PVD layers as well as CVD layers for comparative tests dealt with later on.

Alumina Phase Hardness γ-Al₂O₃ 23 GPa α-Al₂O₃ 20 GPa κ-Al₂O₃ 21 GPa

Orientation Relationships

The single layers of γ-Al₂O₃ were deposited using the following process data on a PVD TiN, Ti(C,N) and (Ti,Al)N layers having a thickness of 3 μm:

T=700° C.

H₂S=1.6%

AlCl₃=2.5%

CO₂=7.0%

H₂=balance

P=100 mbar

Deposition time is 8 hours

TEM micrographs of the PVD TiN-γ-Al₂O₃ interface confirmed the following orientation relationship between PVD TiN and γ-Al₂O₃:

(111)_(PVD-TiN)//(111)γ

[1{overscore (1)}0]_(PVD-TiN)//[1{overscore (1)}0]γ

This orientation relationship is valid for γ-Al₂O₃-PVD TiN, Ti(C,N) or (Ti,Al)N in general and is naturally not dependent on which technique (PVD or CVD) is used to deposit γ-Al₂O₃. The orientation relationship describes in general epitaxy between a face-centered cubic (fcc, preferably fcc B1) coating (TiN, Ti(C,N), (Ti,Al)N) and γ-Al₂O₃ (cubic spinel structure).

Cutting performance in turning of Stainless Steel 1672:

Cutting Speed: 200 m/min

Feed: 0.4 mm/r

Depth of Cut: 2.0 mm

Insert Style: CNMG 120408-M3

Coolant No

Coating (thickness 3 μm on TiCN coating) Average life time/min γ-Al₂O₃ 9.8 α-Al₂O₃ 6.5 κ-Al₂O₃ 9.6

Edge strength/chipping resistance of alumina polymorphs

Turning against shoulder

Cutting Speed: 200 m/min

Feed: 0.4 mm/r

Depth of Cut: 2.0 mm

Insert Style: CNMG 120408-M3

Coolant: No

Coating (thickness 3 μm on TiCN + 3 μm Al₂O₃) Edge chipping after 2 min γ-Al₂O₃ 10% α-Al₂O₃ 15% κ-Al₂O₃ 10%

Edge strength/chipping resistance in milling SS2244

The chipping resistance of PVD TiN, Ti(C,N) and (Ti,Al)N coatings with and without an alumina top layer were studied.

Cutting Speed: 220 m/min

Feed: 0.2 mm/tooth

Depth of Cut: 2.5 mm

Insert Style: SEKN1203

Chipping Coating Thickness after 600 mm Performance/mm PVD TiN 3 5% 3660 PVD Ti(C,N) 3 5% 4200 PVD (Ti,Al)N 3 5% 4600 PVD TiN 6 10% 4700 PVD Ti(C,N) 6 10% 5100 PVD (Ti,Al)N 6 10% 6800 PVD Ti(C,N)-γ-Al₂O₃ 3 + 3 10% 7200 PVD Ti(C,N)-α-Al₂O₃ 3 + 3 30% 7100 PVD Ti(C,N)-κ-Al₂O₃ 3 + 3 25% 5100

It is obvious that γ-Al₂O₃ which can be deposited at lower temperatures than the other CVD alumina phases did not anneal out the compressive stresses in the PVD layers resulting in the better edge strength.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention. 

What is claimed is:
 1. A coated body useful for metal cutting having as the outer layer, a layer of γ-Al₂O₃ deposited by thermal chemical vapor deposition at a temperature of from about 700 to 900° C. and an atmosphere including H₂S in an amount greater than 0.7 vol. %, said body composed of a cemented carbide, cermet or ceramic, wherein the γ-Al₂O₃ layer has a thickness of about 1.0 to 5.0 μm.
 2. The coated body of claim 1 wherein the γ-Al₂O₃ layer is deposited on one or more layers taken from the group consisting of TiC, TiN, Ti(C,N), (Ti,Al)N and Al₂O₃.
 3. The coated body of claim 2 wherein the γ-Al₂O₃ layer is deposited upon one or more layers of α-Al₂O₃, κ-Al₂O₃ or mixtures thereof.
 4. The coated body of claim 2 wherein the γ-Al₂O₃ layer is deposited upon a (Ti,Al)N layer.
 5. The coated body of claim 1 wherein the γ-Al₂O₃ layer is deposited on a layer which has been deposited by physical vapor deposition.
 6. The coated body of claim 5 wherein the γ-Al₂O₃ is deposited on one or more of TiC, TiN, Ti(C,N) and (Ti,Al)N.
 7. The coated body of claim 1 wherein the γ-Al₂O₃ is deposited on a layer having a face-centered cubic structure.
 8. The coated body of claim 7 wherein the γ-Al₂O₃ has an orientation relationship with the layer having a face-centered cubic structure (fcc) of (111)_(PVD-fcc)//(111)γ [1{overscore (1)}0]_(PVD-fcc)//[1{overscore (1)}0]γ.
 9. The coated body of claim 8 wherein the layer having a face-centered cubic structure is taken from the group consisting of TiN, TiC, Ti(C,N), (Ti,Al)N and mixtures thereof.
 10. The coated body of claim 9 wherein the layer having a face-centered cubic structure is a fcc B1 structure deposited by CVD technique.
 11. The coated body of claim 9 wherein the layer having a face-centered cubic structure is a fcc B1 structure deposited by PVD technique.
 12. The coated body of claim 1 wherein the body is a cemented carbide.
 13. The coated body of claim 1, wherein the atmosphere includes H₂S in an amount from 0.75 to 1.7 vol. %.
 14. The coated body of claim 13, wherein the atmosphere includes H₂S in an amount from 1.0 to 1.5 vol. %.
 15. The coated body of claim 1, wherein the temperature is from 750° C. to 850° C.
 16. The coated body of claim 1, wherein the pressure is from 50 to 600 mbar.
 17. A method of forming a coated body having a layer of γ-Al₂O₃ comprising: providing a body to be coated; and coating the body by thermal chemical vapor deposition at a temperature of from about 700 to 900° C. and an atmosphere including H₂S in an amount greater than 0.7 vol. % to form the layer of γ-Al₂O₃, wherein the layer of γ-Al₂O₃ is an outer layer and has a thickness of 1.0 to 5.0 μm.
 18. The method of claim 17, wherein the coating is conducted at a temperature of from about 750 to 850° C.
 19. The method of claim 17, wherein the coating is conducted at a pressure of from about 100 to 500 mbar.
 20. The method of claim 17, wherein the body being coated is a ceramic, cermet, cemented carbide or steel.
 21. The method of claim 20, wherein the body being coated contains at least one other layer of TiC, TiN, Ti(C,N), Ti(Al,N) or Al₂O₃.
 22. The method of claim 21, wherein the said other layer is applied by physical vapor deposition.
 23. The method of claim 17, wherein the atmosphere includes H₂S in an amount from 0.75 to 1.7 vol. %.
 24. The method of claim 23, wherein the atmosphere includes H₂S in an amount from 1.0 to 1.5 vol. %.
 25. A coated body useful for catalysis having as an outer layer, a layer of γ-Al₂O₃ deposited by thermal chemical vapor deposition in an atmosphere including H₂S in an amount greater than 0.7 vol. %, said body composed of steel.
 26. A coated body useful for metal cutting, the coated body comprising: a cemented carbide body; one or more layers deposited on the cemented carbide body, the one or more layers selected from the group consisting of TiC, TiN, Ti(C,N), (Ti,Al)N and Al₂O₃; and a layer of γ-Al₂O₃ deposited as an outer layer by thermal chemical vapor deposition at a temperature of from about 700 to 900° C. and an atmosphere including H₂S in an amount greater than 0.7 vol. %, wherein the γ-Al₂O₃ layer has a thickness of about 1.0 to 5.0 μm. 